When the dinosaurs vanished 66 million years ago, the world's ecosystems were thrown into chaos. For the first few million years afterwards, the largest land animals were no bigger than a dog. Then, as the planet recovered, mammals - which had spent the entire age of dinosaurs as small, mostly nocturnal creatures - rapidly diversified to fill every available niche. By around 45 million years ago, the hoofed predator Andrewsarchus had grown to become the largest carnivorous land mammal that ever lived. Birds also rose to become formidable predators - the giant flightless "terror birds" hunted small mammals across South America. The age of mammals had begun.
Predatory mammals after the extinction ate other mammals - especially the early hoofed plant-eaters that had spread across the new grasslands and forests. Phenacodus, shown here, was an early hoofed herbivore about the size of a sheep. It looked a bit like a small horse with a long tail, lived around 55-50 million years ago, and probably travelled in herds for protection. Andrewsarchus - if its skull size is anything to go by - would have been able to make short work of even larger prey, possibly up to the size of modern cattle.
Phenacodus
It took about 15-20 million years for truly large plant-eating animals to come back after the extinction. Uintatherium appeared in North America and Asia around 45 million years ago and was roughly the size of a rhinoceros. It had three pairs of bony protrusions on its head, downward-curving tusks, and pillar-like legs to support its bulk. Despite its huge tusks, it was a plant-eater, feeding on leaves and fruit. Around the same time, the first "thunder beasts" (brontotheres) appeared in Asia and North America. The largest mammals ever to walk the Earth - Paraceratherium, a 20-tonne hornless rhinoceros relative - lived around 30 million years ago, but were still smaller than the largest dinosaurs.
Uintatherium
Yes - and one of the most remarkable evolutionary stories is the return of mammals to the sea. Ambulocetus ("walking whale"), which lived around 49 million years ago in what is now Pakistan, looks at first glance like a giant otter, but is actually one of the earliest whales. It had short legs and webbed feet, could walk on land, and was a powerful swimmer. Over the next 10 million years, the descendants of Ambulocetus gradually lost their hind legs, lengthened their bodies, and became fully aquatic - eventually giving rise to the whales and dolphins we know today. Modern whales still have tiny leftover hip bones from their land-walking ancestors.
Ambulocetus
After the dinosaurs died out, several lineages of birds evolved into formidable land predators. The most spectacular were the phorusrhacids - the "terror birds" - which dominated South America for millions of years. Titanis, one of the largest, was about 2-2.5 metres (6.5-8 feet) tall, with a huge hooked beak the size of a horse's head and powerful clawed feet. Terror birds could not fly, but they could run fast - estimates suggest 50 kmh (30 mph) or more - and they tore prey apart with their beaks like flightless dinosaurs from a forgotten age. They eventually went extinct around 1.8 million years ago, possibly outcompeted by mammalian predators that crossed from North America.
Titanis
Bats were among the first mammals to take to the air after the dinosaurs vanished. The earliest fully developed bat fossils, including Icaronycteris from Wyoming, USA, are about 52 million years old. Even by that early date, bats were already wonderfully adapted for flight: they had long arm and finger bones supporting a membrane wing, much like modern bats. Today, bats remain the only mammals capable of true flight. They make up roughly one in every five mammal species on Earth - and many use echolocation to hunt insects in the dark, an ability that may have evolved well over 50 million years ago.
Icaronycteris